1. Field of the Invention
This invention relates in general to downhole drilling motors of the progressive cavity type.
2. Description of the Prior Art
Downhole drilling motors have been used for many years in the drilling of oil and gas wells. In the usual case, the shaft of the motor and the drill bit will rotate with respect to the housing of the drilling motor. The housing is connected to a conventional drill string composed of drill collars and sections of drill pipe. The drill string extends to the surface, where it is connected to a kelly, mounted in the rotary table of a drilling rig. Drilling fluid is pumped down through the drill string to the bottom of the hole, and back up the annulus between the drill string and the wall of the bore hole. The drilling fluid cools the drilling tools and removes the cuttings resulting from the drilling operation. If the downhole drilling motor is a hydraulic motor, the drilling fluid also supplies the hydraulic power to operate the motor.
One type of hydraulic downhole motor is the progressive cavity type, also known as the Moineau motor. These devices are well known in the art and have a helical rotor within the cavity of a stator, which is connected to the housing of the motor. As the drilling fluid is pumped down through the motor, the fluid rotates the rotor. As the helical rotor rotates, it also gyrates, or orbits, in the reverse direction relative to its rotation. Some type of universal connection must be used to connect the gyrating rotor to the non-gyrating shaft of the motor.
One type of connector utilizes a pair of universal joints which connect a straight rod to the rotor and to the shaft. The universal sections are designed to take only torsional load. A ball and race assembly is used to take the thrust load. Rubber boots are clamped over the universal sections to keep drilling fluid out of the ball race assembly. Most assemblies of this type also require oil reservoir systems to lubricate the ball race and universal joints. Problems exist with the rubber boot systems. Boots may loosen and come off, allowing drilling fluid to enter and wear out the ball race assembly. That forces the universal joints to take torsional and trust loads, causing premature failure. Other motors have had long, flexible shafts, which flex to compensate for the gyration of the rotor. However, when these shafts are long enough to provide sufficient flexing, the overall length of the motor is excessive. A need existed for a connecting rod which was sufficiently flexible, without being excessively long.